With the demand for low fuel consumption for automobiles in recent years, there has been demand for a conjugated diene-based polymer, as a rubber material for a tire, having low rolling resistance, excellent abrasion resistance and tensile properties, and adjustment stability exemplified by resistance on wet roads.
In order to reduce the rolling resistance of the tire, there is a method of reducing the hysteresis loss of the vulcanized rubber. As the evaluation index of such vulcanized rubber, rebound resilience at 50 to 80° C., tan δ, Goodrich heating or the like are used. That is, a rubber material having high rebound resilience at the above-described temperature or a low tan δ and low Goodrich heating is preferred.
Natural rubbers, polyisoprene rubbers, polybutadiene rubbers, and the like are known as rubber materials having a small hysteresis loss, but these rubbers have low resistance on wet roads. Therefore, recently, a conjugated diene-based polymer or copolymer such as styrene-butadiene rubber (hereinafter referred to as SBR) or butadiene rubber (hereinafter referred to as BR) has been prepared by emulsion polymerization or solution polymerization and used as a rubber for a tire. Among these, the greatest advantage of solution polymerization over emulsion polymerization is that a vinyl structure content and styrene content, which define rubber properties, can be arbitrarily controlled, and a molecular weight and physical properties can be controlled by coupling, modification or the like. Therefore, since it is possible to easily change the structure of the finally prepared SBR or BR, reduce the movement of chain ends by bonding or modification of chain ends, and increase bonding force with a filler such as silica or carbon black, SBR prepared by solution polymerization is widely used as a rubber material for a tire.
When the solution-polymerized SBR is used as a rubber material for a tire, the glass transition temperature of the rubber can be raised by increasing a vinyl content in the SBR such that required properties of a tire such as running resistance and braking force can be controlled. Fuel consumption can also be reduced by suitably controlling the glass transition temperature. The solution-polymerized SBR is prepared by using an anionic polymerization initiator, and chain ends of the polymer formed are bonded or modified by using various modifiers. For example, U.S. Pat. No. 4,397,994 discloses a technique in which an active anion at the chain end of a polymer, obtained by polymerizing styrene-butadiene in a nonpolar solvent using alkyllithium, which is a monofunctional initiator, is bonded using a binder such as a tin compound.
Further, carbon black and silica are used as reinforcing fillers for tire treads. When silica is used as a reinforcing filler, hysteresis loss is low and resistance on wet roads is improved. However, as compared to carbon black having a hydrophobic surface, silica having a hydrophilic surface has a disadvantage in that affinity with rubber is low and dispersibility is poor. Therefore, it is necessary to use a separate silane coupling agent in order to improve the dispersibility or to provide the bond between the silica and the rubber. Thus, a certain method of introducing a functional group having affinity or reactivity with silica to the terminal of the rubber molecule has been proposed, but the effect is insufficient. Further, even when a reinforcing filler is used together with the solution-polymerized SBR, abrasion resistance still does not satisfy the required level.